Light weight construction with high strength steels enables costs and recourses efficient products. The strength of steels can be increased though solid solution hardening, conversion hardening, fine grained hardening, precipitation hardening and cold forming. Usually, all these mechanism provide a contribution to the component strength.
Known is the so-called “steel-banana”, which illustrates the relationship of strength and elongation at break for different steel grades. This is shown in FIG. 1 (Source: ADVANCED HIGH STRENGTH STEEL (AHSS) APPLICATION GUIDELINES Version 4.1, www.worldautosteel.org).
In the documents “A. J. DeArdo and R. A. Walsh, “High strength low alloy steel”, U.S. Pat. No. 5,352,304 A04-Okt-1994” and “E. J. Czyryca, “Development of law-carbon, copper-strengthened HSLA steel plate for naval ship construction”, DTIC Document, 1990”, high strength Cu alloyed steels are described, which are preferably used in ship construction in a hot rolled and hardened condition. The steel is additionally alloyed with nickel in order to reduce the susceptibility to red shortness and to increase the ductility. Such high nickel contents are however not applicable in many markets and applications for financial reasons.
The possibility of the use of Cu for the increase in strength of IF-steel is described in the document “R. Rana, W. Bleck, S. B. Singh, and O. N. Mohanty, “Development of high strength interstitial free steel by copper precipitation hardening”, Mater. Lett., ed. 61, No 14, p. 2919-2922, 2007”. By addition of copper, high strengths of 500-600 MPa are reached. The document “R. Rana, S. B. Singh, and O. N. Mohanty, “Effect of composition and pre-deformation on age hardening response in a copper-containing interstitial free steel”, Mater. Charact., ed. 59, no 7, p. 969-974, July 2008” concerns investigations about the influence of the deformation on the precipitation and strengthening behavior of a Cu alloyed IF-steel. According to that, the precipitation hardening is accelerated though a deformation of more than 40%. The document “N. Maruyama, M. Sugiyama, T. Hara, and H. Tamehiro, “Precipitation and phase transformation of copper particles in low alloy ferritic and martensitic steels”, Mater. Trans.-JIM, ed. 40, p. 268-277, 1999” discloses that the strength of a Cu alloyed ferritic, cold work hardened material increases having an annealing treatment between 700 and 900 kelvin. The influence of copper addition is described in the document “R. Rana. W. Bleck, S. B. Singh, and O. N. Mohanty, “Hot shortness behavior of a copper-alloyed high strength interstitial tree steel”, Mater. Sci. Eng. A, ed. 588, p. 288-298, Dez. 2013”.
The document “D. isheim, M. S. Gagliana, M. E. Fine, and D. N. Seidman, “Interfacial segregation at Cu-rich precipitates in a high-strength low-carbon steel studied on a sub-nanometer scale”, Acta Mater., ed. 54, no 3, p. 841-849, February 2006” describes the use of a atom probe (APT Atom Probe Tomography) for the analysis of the Cu precipitation of a high strength steel for ship building applications. Cu precipitations in steel are not detectable with conventional light microscopic and scanning electron microscopic methods, because the size of the precipitated particles lies in the area of a few nanometers.
A tensile test for metallic materials is a to DIN EN ISO 6892 standardized material testing standard method. According to the invention, the material parameters are preferably also measured according to this DIN.
An elongation at break A is a material parameter, which shows the remaining elongation of a sample after the break in relation to the initial measuring length. It characterizes the ductility (deformation capability) of a material. It is the remaining length difference ΔL after occurred breakage in relation to the initial measuring length L0 of a sample in the tensile test.A=ΔL/L0·100%
The initial length L0 is defined by means of measuring marks on the tensile specimen prior to the tensile test. Due to the locally limited necking, the elongation at break A is depending from the initial measuring length L0. In order to obtain comparable values for the elongation at break, most of the time proportional sticks are used for the tensile tests, i.e. specimen having the initial measuring length L0 being in a fixed relation to the initial cross section S0.L0=k·d0 
For round rods, a value of K=5 is common. The elongation at beak is then called A5.
During tensile testing, a local necking occurs with ductile materials after reaching the tensile strength Rm, wherein in this area then also occurs the break. The thereby biggest occurring relative change in cross section is called necking (reduction in area at break or failure) Z. This is a measure for ductility of the material:
                              Z          =                    ⁢                                                                      Δ                  ⁢                                                                          ⁢                  S                                                  S                  0                                            ·              100                        ⁢            %                                                        =                    ⁢                                                                                          S                    0                                    -                                      S                    u                                                                    S                  0                                            ·              100                        ⁢            %                                                        =                    ⁢                                                    (                                  1                  -                                                            S                      u                                                              S                      0                                                                      )                            ·              100                        ⁢            %                                 with the initial cross section area S0 of the unloaded specimen rod and the smallest cross section area Su of the broken rod, thus the remaining cross section area at the place of necking.
The tensile strength is the tension, which is calculated at a tensile test from the maximal reached tensile force in relation to the original cross section of the specimen. As formal sign of the tensile strength is for example used the expression Rm. Dimension of the tensile force is force per area. Often used measurement units are N/mm2 or MPa. The tensile strength is often used for the characterization of materials.
The uniform elongation Ag is at the tensile test the plastic length change Lpm-L0 in relation to the initial length L0 at loading the tensile specimen with the maximal force Fm. This is mostly reached at the tensile strength Rm. The uniform elongation Ag indicates that the tensile specimen does not reveal necking until the maximum force, but stretches uniformly.
      A    g    =                                                        L              pm                        -                          L              0                                            L            0                          ·        100            ⁢      %        =                            (                                                    L                pm                                            L                0                                      -            1                    )                ·        100            ⁢      %      
The yield strength Re is a material parameter and designates that tension, to which a material has no permanent plastic deformation at uniaxial and moment free tensile load. This refers to a yield point. At exceeding the yield strength, the material does not return anymore back to the original shape, but a specimen prolongation remains. The yield strength is commonly determined though tensile test.
Increasing importance gain components/constituents made of steel materials in a tensile strength range between 600 MPa and 1200 MPa. The steel-banana (s. FIG. 1) is characterized in that the product of tensile strength and elongation at break is roughly equal for many steel grades and lies for common low cost steels with ferritic, pearlitic, bainitic or martensitic matrix at about 15000 MPa*%. The product of tensile strength (Rm) measured in a quasi static tensile test in MPa and elongation at break (A) in % can taken as simple quality criterion for a steel, wherein attention need to be paid in detail on the different criteria for measuring the elongation at break. Usually, the total elongation at break A5 consisting of a portion of uniform elongation and a portion of necking elongation forms the basis for this comparative illustration.
An increase of the ductility at constant strength enables one the one side a higher energy absorption at overload (component safety), and on the other side there is the potential for further cold working steps such that more complex components can be manufactured.
A design target for the manufacturing of new, high strength steels is therefore often the increase of this product of Rm*A. Results of such efforts is for example TRIP steels and TWIP steels with a product of Rm*A of sometimes more than 50000 MPa*%. For different reasons, e.g. costs and difficulties during processing, the practical use of such steel grades is presently limited though.
In FIG. 1, MILD means commonly deep drawing grades, BH “bake hardening steel”, i.e. higher strength steels with yield strength increase by means of the paint baking, IF “interstitial free steel”, i.e. steel without interstitial dissolved alloy contents, HSLA “high strength low alloy steel”, i.e. high strength low alloyed steel, TRIP “transformation induced plasticity steel”, i.e. steel with though crystal lattice transformation induced plastic deformation, TWIP “Twinning induced plasticity steels” DP-CP dual phases/complex phases with soft ferrite, MS martensitic phase steel, IS isotropic steel, IF-HS higher strength IF steel, CMn manganese-carbon-steel (common structural steel).
Semi-finished good is the general term for pre-manufactured raw material shapes like for example sheets, rods, pipes and coils. In the manufacturing technology, semi-finished goods represent by far the most spread delivery form for metal materials. It is distinguished between over 1000 sorts of semi-finished goods made of metal and plastic, which each are standardized in terms of material and surface quality, shape and dimensions as well as their tolerances. Typically for semi-finished goods is that the first processing step consists of a cutting, wherein the needed material section is cut off by means of a suitable method (e.g. sawing). This material section is further processed to the actual finished part.
High geometrical precision of semi-finished goods and finished products are reached through a cold working, e.g. cold rolling of strip material, cold drawing of pipes, cold heading of rod material, thread rollers, etc. By means of the cold working, also the strength of the semi finished goods and products highly increases. A significant cold working generally results in that the product of tensile strength and elongation reduces dramatically due to the drastically reduction of the ductility.
A cold worked steel with a strength of 1000 MPa and a resulting elongation of 2% has for example only a product Rm*A of 2000 MPa %.
Nevertheless, the use of cold working of cost efficient low alloyed steel grades has a high importance to an economic light weight construction in different applications.
Along with the strength increase is however related disadvantageously also a considerable reduction of ductility such that a further plastic deformation in following processes is not directly possible. Furthermore, in many cases plastic deformability is also requested for the component safety in order to enable depending on the application a more or less high energy absorption before a break.
For adjustment of an improved ductility after the cold working, the steel commonly undergoes a heat treatment. During annealing under a shielding gas (e.g. nitrogen, argon) in an area between 600 and 700° C. above the recrystallization temperature recrystallizes the material, the cold work hardening is mostly removed and a high plastic deformability is obtained. This process is usually called normalizing.
Also industrial practice is a stress relief annealing below the recrystallization temperature. During annealing below the recrystallization temperature, a so-called crystal regeneration occurs with steels. Crystal regeneration leads to reduction of stress. Grain shape and grain size of the deformed microstructure are preserved. The reduction of inner stress is connected with a moderate increase of the ductility and a moderate reduction of strength. The process window of such a stress relief annealing is comparatively narrow, because the temperature usually has to be brought close to the recrystallization in order to reach a considerable increase of the ductility. This temperature is furthermore not only material dependent, but also dependent from the pre-deformation. Thanks to the stress relief annealing the ductility can in fact be increased, but the product of Rm*A commonly only reach values below 10000 MPa %. If high values of Rm*A at high strength should be reached, there is moreover an extensive risk due to the narrow process window for fluctuating good (product) properties, which can result in an increased waste.
The heat treatment of cold hardened semi-finished goods is conducted for strip material in flow-through annealing systems and stationary bell type annealing systems. Depending on the furnace loading, only small heating up and cooling down speeds can be reached. The bell type annealing of strip material in form of so-called coils thus require up to several days due to the physical related soak time. A heat treatment of the material, i.e. transformation hardening and tempering, is in general technically not possible having these slow cycles. Furthermore, the transformation hardening is disadvantageous connected with high temperatures and energy use, high costs as well as the risk of distortion of the semi-finished goods.
Therefore, a high strength and at the same time a high deformability at cold worked semi-finished goods and products are only to be reached with difficulties. In particular, at low alloyed, cold worked steels, a strength of more than 750 MPa and a elongation at break A of more than 15% are until now hardly reached in a process safe manner in the chain of cold working and stress relief annealing.#
As a result, there is a technological gap in the strength area 700 MPa to 1200 MPa for bell type annealed cold strip with high ductility. This also applies for pipes and rod materials, which is processed e.g. though cold drawing, cold heading, thread rolling, if heat treatment (transformation hardening and tempering) has to be waived for financial and technological reasons, e.g. occurrence of distortion.